The explanation of the terms listed below refer to their usage in the field of Mechanical Music. Many of the descriptions are from the book The Golden Age of Automatic Musical Instruments by MBSI member Arthur A. Reblitz, published by the Mechanical Music Press, Woodsville, New Hampshire; copyright 2001. They are used here with permission of the author and publisher. Permission is granted for writers to use a limited number of brief excerpts provided credit is given to the MBSI web site, the title of the original work and the author. Written permission is required for all other uses. German nouns and words that are also names of places are capitalized.

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Una-Fon bells: Bar-type bells, usually with a scoop-like depression in the center, with a closed-end tubular resonator behind each bar, made by J.C. Deagan. The Una-Fon is a hand-played instrument with the bells played by reiterating electromagnets fitted with hard beaters. Due to their unusual shape, resonator length and hardness of the beaters, the bells have a unique sound with a prominent harmonic clang tone. Una-fon bells, also called Unitone bells by Deagan, were used in certain early Cremona Style J orchestrions (as Unatone bells) and Wurlitzer 180 band organs (as Uniphone bells).

unda maris: Literally, "waves of the sea." Two ranks of celeste-tuned string pipes used in dance organs and street organs, usually in the melody section, tuned an octave below the melody violins. The unda maris pipes produce a characteristic undulating sound from which the "waves of the sea" name (in Latin) is derived.

unification: Or unified system. System used in most theatre organs and in some smaller instruments (such as certain mortuary organs made by Seeburg and Operators) whereby the keyboard may be connected to the rank of pipes in several different ways. If the 8' flute stop is selected, for example, the keyboard is connected to the rank of flute pipes so they play at normal pitch. If the 4' flute stop is selected, the pipes play an octave higher. If the 16' flute stop is selected, they play an octave lower. Or, by selecting the 16', 8' and 4' flute stops at the same time, three pipes in the same rank play when each key is played. In contrast, a straight (or non-unified) organ has a separate rank of pipes for each stop. Because each rank of pipes in a unified organ is available at several pitches on several different manuals, a unified organ has fewer ranks of pipes in proportion to the number of stops.

unit valve: Type of pneumatic stack construction with individually-removable valve units. Each valve and pouch (and sometimes pneumatic) is contained within one unit, making servicing of defective notes easier when factory replacement units were available. Today, restoring a unit valve pneumatic stack frequently takes longer than restoring a stack made in one big chest, as there are many parts to break open for restoration instead of one big unit that usually comes apart with screws.

Univox: Small electronic organ used in some Decap and Mortier/van den Bosch dance organs built in the 1950s.

upright piano: Large piano in which the strings and soundboard are mounted vertically. In descending order of size, vertical pianos are called uprights, studio uprights, consoles, and spinets.